RE: BRK opcode

From: Gideon Zweijtzer (
Date: 2003-11-05 06:47:00

> Everywhere I look (almost everywhere then) it lists BRK as a 
> single byte opcode but in operation it is 2. Is there some 
> official explaination? I notice most assemblers treat it as a 
> single byte and maybe causing headaches for many users.
Hello Nathan,

Of course, only the design team of the 6502 knows for sure what the
reason is, but the reason that I found out is that the BRK instruction
is just on a 2-byte size location in the instruction map. This is the
rule that I use for my own processor implementation. The size of a 6502
instruction is easily derived from the opcode by the following formula:
(The result is a two-bit value, where "00" is one byte, "01" is two
bytes and "10" is three bytes):

Bit1: xxxx11xx or xxx110x1
Bit0: xxxx0xxx or xxx010x1

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